1. Field of the Invention
The present invention relates to a method and apparatus for displaying images while encoded moving data is being decoded.
2. Description Of the Related Art
Before an explanation is given about the prior art, the technical background related to the present invention will be explained. In recent years, moving pictures have come to be encoded and stored in a storage medium. Standardization of such an encoding system is now under way (Reference: The Journal of the Institute of Image Electronics Engineers of Japan, Vol.19, No.4, (1990), pp.236 to 243; Technical Description: International Standardization for Coding of Moving Images, Audio and Multimedia Synchronization by Yamada and Onoe). In this standardization, an intended application is to make it possible to display a stored moving image on a display device. In this standardization, motion compensated adaptive inter-frame prediction is made the basis, prediction errors are transformed by a two-dimensionally discrete cosine transformation (DCT), adaptive and weighted quantization is performed on obtained coefficients, these coefficients are made into a one-dimensional data sequence by zigzag scanning, and this data is encoded by entropy encoding. To improve the quality of displayed images, the frames which are used for encoding only within a frame (called an intra-frame) in a fixed or adaptive manner are inserted. To achieve high-speed reproduction, the frames used for inter-frame prediction (called predictive frames) are inserted at intervals of several frames. The frames between the predictive frames are made into interpolative frames by which motion compensation is performed on the basis of succeeding and preceding predictive frames.
The state of such frames is shown in FIG. 3. In this figure, intra-frames (reference numerals 301 and 302) appear at N frame intervals. In between the N frame intervals, predictive frames (reference numerals 303 and 304) appear at 3 frame intervals. The other frames are interpolative frames. In this figure, each frame is encoded with a block formed of 8.times.8 pixels as a unit. In the intra-frames, two-dimensional DCT is performed on 8.times.8 blocks. Weighted quantization is performed on the coefficients. In the predictive frames, two-dimensional DCT is performed by considering the error and the prediction block obtained by performing motion compensation on the basis of the immediately preceding predictive or intra-frame as one block, and weighted quantization is performed on the coefficients. In the interpolative frames, an error among prediction blocks, obtained when performing forward and backward motion compensation and when interpolating both compensations, is determined on the basis of the succeeding and preceding predictive or intra-frames. The smallest error among the blocks of the compensation system is two-dimensionally DCTed, and weighted quantization is performed on the coefficients. Respective quantization results obtained in the above-described way are zigzag scanned, and the obtained one-dimensional information is entropy encoded. Thus, encoded data is obtained.
Therefore, in the intra-frames, a code indicating the result of the quantization of each block is provided after a code indicating the intra-frame. In the predictive frames, a motion vector of each block and a code of the quantization result are provided after a code indicating the predictive frame. In the interpolative frames, a code indicating the motion compensation means selected for each block and a code of the quantization result are provided after the code indicating the interpolative frame.
Generally in the interpolative frames, the image quality decreases because the length of the code is shortened when the quantization width is made larger. Therefore, the amount of encoding allocated during encoding is largest in the intra-frames, the next largest being in the predictive frames. The frames having the smallest number of bits are the interpolative frames.
However, in the above-described prior art, when a moving image is made temporarily still and displayed, images stored in the video frame memory cannot be displayed. In addition, when the display is made temporarily still in the interpolative frames, a problem arises in that the image deteriorates conspicuously because an image of lower quality than that of the other frames is displayed.